Other Worlds: Farewell to Cassini

This week’s guess the planet image comes from Saturn and shows the swirl of clouds around the north pole of the distant gas giant. This image was taken by the Cassini Spacecraft as it began its final descent into Saturn. Today the Cassini mission comes to an end, as the spacecraft crashes into the planet it has spent more than a decade studying. This is the end of an era, and it will be a long time before we have another probe to send back marvellous pictures of Saturn and its moons. I want to look back at this fantastic spacecraft, and some of the things it has accomplished over the course of the 13 years at Saturn.

The Cassini-Huygens mission was launched in 1997 as a joint operation between NASA, who built the orbiter, and ESA who constructed the Huygens lander. It spent the next seven years travelling to the outer solar system, and the vicinity of Saturn and its moons. Unlike the Voyager probes which had flown past Saturn on their way to the worlds beyond, Cassini was there to stay and went into orbit around the gas giant in 2004. This allowed it to study Saturn in unprecedented detail, and for far longer than any fly by mission could. Cassini was placed into an elliptical orbit which would allow it to also perform regular flybys of several of Saturn’s moons, including Titan and Enceladus.

Titan was of particular interest to the mission, as it was the destination of the Huygens lander. This small probe touched down on the surface of Titan in January of 2005. This was the first time a lander had touched down on the surface of such a distant moon, and it sent back a lot of valuable data about the surface conditions on Titan. This moon has a thick atmosphere, which has shrouded the surface, hiding its features from view. In addition to deploying the Huygens lander Cassini also used Radar to peer through those clouds and return satellite images of the surface. In doing so it discovered vast hydrocarbon lakes and rivers, which we’ve talked about before on this blog.

The Surface of Titan, from the Huygens Lander.

Cassini’s observations of Enceladus were also very valuable. The probe detected a thin atmosphere of ionised water vapour, and observed the geysers that periodically erupt from the small moon.

By flying through these geysers it was able to determine that they contain organic compounds from the subsurface ocean beneath the icy world. This is significant because, as the name suggests, organic compounds are a vital precursor to life. If organisms like those on Earth are to evolve in an extraterrestrial environment it will have to be one with organic compounds, so this discovery makes Enceladus and moons like it a prime target for astrobiological study. The presence of organic compounds doesn’t necessarily mean that life will evolve there, but it gives it a better chance.

Cassini didn’t stop at investigating the moons which we already knew about, but discovered six more during its time at Saturn. Naturally it also made numerous observations of Saturn’s famous ring system, including observing spoke like patterns in the ring system which had previously been detected through telescopes and by the Voyager probes. It made numerous observations of the structure of the rings, and the sizes of the particles that comprise them.

Cassini also turned its attention to the atmosphere of Saturn, observing storms in the gas giant, and studying the composition of the atmosphere. It observed the “great white spot” storm that recurs every 30 years at Saturn, and has observed a stable hurricane at the planet’s South Pole.

The Cassini mission has encompassed far too many discoveries to cover them all in detail here. In the 13 years it has spent at Saturn it has massively expanded our understanding of this distant world. The sailing hasn’t always been smooth, in particular there were communication problems surrounding the Huygens landing, which required the ingenuity of the team behind the spacecraft to solve. However, despite the occasional setback, Cassini has been a dependable spacecraft for over a decade, sending back the most, and best, data we have ever had about this distant world.

Cassini will continue to record data as it plunges into the atmosphere of Saturn, although NASA do not expect that much of this will be received. Nonetheless the run up to the spacecraft’s destruction has allowed the team to perform multiple close flybys of the rings, the inner moons and the planet itself. The “grand finale” of the Cassini mission has already been a spectacular show.

Cassini will be sorely missed, but the contributions it has made will keep planetary scientists busy for decades!